Accretion of sulfate or methionine sulfur by the rat fetus is depressed by administration of drugs, such as salicylamide, which undergo sulfoconjugation. The aims of the present investigation were: 1. To follow after injection into the pregnant rat, the sequential uptake and loss of S35-sulfate and S35-methionine by maternal serum, placenta and fetus. 2. To determine the effect of salicylamide on these parameters. 3. To show whether salicylamide exerts an adverse effect on the development of the rat fetus. The rate of change of placental and fetal radiosulfate uptake with time was found to be dependent on the maternal serum level. Salicylamide decreased radiosulfate uptake by maternal serum, placenta and fetus; effects being dose dependent. Differences induced by salicylamide on radiosulfate uptake by the placenta and fetus persisted when correction was made for the effects of maternal serum sulfate level. S35-methionine uptake by the rat fetus and placenta was also found to be dependent on maternal serum level, though the fall off in maternal serum was not reflected in a corresponding fall-off in the radiosulfur level in placenta or fetus. Fetal malformations, including micromelia and short tail deformity, occurred in salicylamide treated animals, the incidence being related to the dose of the drug received.